1. Technical Field
Example embodiments relate to a semiconductor device and to a method of manufacturing the same and, more particularly, to a semiconductor device having elevated source/drain (ESD) structures, and to a method of manufacturing the same.
2. Description of the Related Art
Due to the high degrees of integration of recent semiconductor devices decreased design rules for various elements of the semiconductor devices may now be required. However, the decreased design rules may cause transistors to have short channels and small contact areas between the transistors and various conductive structures. Particularly, the operational characteristics of a semiconductor memory device may be significantly influenced by a short channel and a small contact area of a transistor, because the semiconductor memory device may include a large number of transistors.
A short channel of the transistor may cause a short channel effect in the transistor, and thus a leakage current may be increased in the transistor and a refresh time may not be sufficiently obtained in a memory device. The small contact area may in turn cause contact resistance to increase at both the source electrode and the drain electrode of the transistor, and cause the process margin of a critical dimension to be reduced, to thereby generate a contact failure.
Consequently, various types of research have been conducted for enlarging the contact area at source/drain regions so as to reduce contact resistance at the source/drain regions. As a result of the above-mentioned research, an elevated source/drain (ESD) structure has been suggested and most widely used for enlarging a contact area at source/drain regions. A conventional ESD structure may include a stacked gate electrode having a spacer at both side walls thereof and epitaxial layers selectively grown at surface portions of an active region adjacent to the gate electrode.
However, as a conventional selective epitaxial growth (SEG) process is an isotropic process, the conventional ESD structure grows at the same rate both in a vertical direction perpendicular to the substrate and in a horizontal direction parallel with the substrate. Accordingly, the conventional ESD structure may have difficulties associated therewith in that the device isolation layer may be covered with the ESD structures and the source and drain regions of the active region adjacent to each other may not be electrically separated from each other.
FIG. 1 is a plan view illustrating a semiconductor device including a conventional ESD structure. In FIG. 1, a dash line indicates an active region before the source/drain regions are elevated and a dotted line indicates an active region of which the contact area is enlarged due to the elevation of the source/drain regions.
Referring to FIG. 1, a gate structure (not shown) is formed on a semiconductor substrate 10 in which a gate oxide layer and a gate stack are stacked on the substrate 10 and a gate spacer is formed at both side walls of the gate stack. The substrate 10 includes an active region 12 in which a conductive structure is positioned as a unit device for an electric circuit and a device isolation region 14 by which adjacent active regions are electrically isolated from each other; and thus the conductive structures on the adjacent active regions may be electrically isolated from each other. In general, an insulation layer (not shown) is formed in the device isolation region 14 by a shallow trench isolation (STI) process, to thereby form a device isolation layer (not shown) in the device isolation region 14 for electrically isolating the conductive structures on the adjacent active regions 12 from each other.
The source/drain regions adjacent to the gate structure are enlarged to an expanded area 12b from an original area 12a by a SEG process, and thus the contact area may be enlarged and the contact failures may be significantly reduced in the semiconductor device.
A SEG process is an anisotropic process, and thus a SEG layer may not be formed on the device isolation layer comprising oxide or nitride. However, as the design rule of a semiconductor device is decreased, the SEG process may be gradually transformed into an isotropic process in place of an ideal anisotropic process, and thus the device isolation layer may be partially covered with an epitaxial layer.
That is, the epitaxial layer of the ESD structure extends to an upper portion of the device isolation layer defining the active regions, so that the source/drain regions of the adjacent gate structures are not electrically separated from each other by the device isolation layer. Therefore, when contact plugs are formed on the ESD regions, adjacent contact plugs may be electrically connected to each other, and thus the adjacent source and drain regions may be electrically connected to each other, to thereby generate a 2-bit failure in the semiconductor device.
FIG. 2A is a cross-sectional view taken along a line I-I′ of a portion A of the semiconductor device shown in FIG. 1, and FIG. 2B is a cross-sectional view taken along a line II-II′ of a portion B of the semiconductor device shown in FIG. 1.
Referring to FIG. 2A, when the adjacent gate structures 20a and 20b are spaced apart from each other by a sufficient gap distance, epitaxial layers 30 grown at the source and drain regions may be sufficiently separated from each other to thereby form an elevated source region 34 and an elevated drain region 32 on the substrate 10. Thus, the elevated regions 34 and 32 at the source and drain regions may sufficiently function as source and drain regions for the adjacent gate structures 20a and 20b. A conductive structure such as a contact plug is individually formed on the elevated source and drain regions 34 and 32 and each of the conductive structures may be electrically operated independently from each other.
However, as shown in FIG. 2B, when the adjacent gate structures 20a and 20b are not sufficiently spaced apart from each other, the epitaxial layer is grown to cover the whole surface of the device isolation layer, and thus the device isolation layer may be fully covered with the epitaxial layer 30. Thus, the elevated drain region 32 for the first gate structure 20a may not be separated from the elevated source region 34 for the second gate structure 20b, and each of the contact plugs located on the elevated source and drain regions 34 and 32, respectively, may also electrically separated from each other, to thereby generate a bridge defect. As a result, the first and the second gate structures 20a and 20b may not be electrically separated from each other to thereby generate the 2-bit failure in the semiconductor device.
Accordingly, there is still a need for an improved method of forming an ESD structure in which lateral growth is sufficiently prevented on a surface of the substrate to thereby reduce contact failures such as the bridge defect.